The present invention relates generally to magnetic resonance imaging (MRI) systems and, more particularly, to a system and method for receiving and directly digitizing imaging data signals. The invention is capable of utilizing commodity analog-to-digital (A/D) converters and adaptable software-processing algorithms to improve image quality while reducing manufacturing costs.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment, Mt. A signal is emitted by the excited spins at or around the Larmor frequency after the excitation signal B1 is terminated that is then received and processed to form an image. The Larmor frequency at a 1.5 Tesla (T) polarizing field strength is around 64 MHz.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MRI signals is received using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
To transmit the RF signals that are used to excite the desired spins and receive the resulting MRI data signals, transmit and receive coils or a common transceiver coil is used. The receiver coil (or transceiver coil) receives the MRI data signals excited during the imaging process and provides the data signals to various hardware processing components.
In particular, the signal produced by the subject being imaged in response to excitation by the RF excitation pulses is picked up by a receiver coil and applied through a preamplifier to a receiver amplifier. The receiver amplifier further amplifies the signal by an amount determined by a digital attenuation signal received from the pulse sequence server. Since the received signal is at or around the Larmor frequency and these hardware-based receiver systems cannot provide adequate sampling at such high frequencies, this high frequency signal is down-converted in a two-step process by a down converter that first mixes the imaging signal with the carrier signal and then mixes the resulting difference signal with a reference signal. In this regard, these hardware systems typically down convert the received analog signals to an intermediate frequency that is less than the MRI imaging frequency and then mix it with an analog reference signal.
Only after this conversion and mixing is the signal finally digitized by an analog-to-digital (A/D) converter that samples and digitizes the down-converted/mixed analog signal. Once digitized, the signal is applied to a digital detector and signal processor that produces 16-bit in-phase “I” values and 16-bit quadrature “Q” values corresponding to the received signal. Therefore, only after a variety of significant analog processing steps are the analog signals finally digitized and processed to reconstruct the resulting image.
To carry out these mixing and digitizing processes, hardware systems are employed that are specifically tailored to the particular MRI system with which the mixing and digitizing hardware is to be associated. For example, once the constraints of a particular MRI system are identified (i.e., 1.5 Tesla or 3 Tesla and capable of only echo-planar imaging processes or capable of other fast-spin-echo techniques, such as gradient- and spin-echo processes), hardware that is specifically designed to prepare (i.e., synchronize and digitize) the imaging data received under those constraints is coupled therewith. That is, the hardware is specifically designed and tailored to perform down-conversion, mixing, and analog-to-digital conversion under the specific constraints and parameters (i.e., sampling frequency and MRI frequency) necessary for a given MRI system.
While these hardware-based systems yield suitable results, they are extremely rigid since they are specifically designed and tailored for a particular MRI system. In this regard, although a wide variety of components, such as analog-to-digital converters, are produced as commodity (i.e., low-cost and/or mass-produced) components for use in mass-market devices (e.g., cellular phones and the like), these components cannot be readily utilized in MRI systems without redesigning or reconfiguring the hardware of the receiver system to accommodate the specific functionality of a given commodity component. Furthermore, as various hardware designs and components attain higher bandwidth and dynamic range, these MRI systems cannot harness these capabilities to yield higher quality images without hardware-level redesigns and reconfigurations of the receiver system.
Accordingly, the original manufacturers of MRI systems cannot readily adapt to the fast-paced and ever-changing world market of commodity components because in-depth hardware redesigns and reconfigurations would be required for each and every new chip or board that is selected. Accordingly, though commodity components might provide significant manufacturing savings, the design costs associated with adapting to varying component constraints preclude the realization of such savings.
Similarly, end users cannot simply replace the hardware-based receiver components that were originally included in an MRI system with new components that yield higher bandwidth and/or dynamic range. In this case, though an end user may wish to reap the benefits of newly available components that yield improved image quality, the constraints of the hardware-based receiver system preclude the integration of after-market components into an MRI system.
Therefore, it would be desirable to have a system and method for facilitating the adaptability necessary to accommodate changing component constraints. Furthermore, it would be desirable to allow end users to upgrade the receiver system of a given MRI system to improve image quality without undue reconfiguration and redesign. Additionally, it would be desirable to provide a system and method to facilitate direct detection of MRI imaging data. That is, it would be desirable to have a system and method capable of receiving and digitizing MRI imaging data signals without the need for intermediate analog processing steps, such as intermediate frequency processing.